VTE: Fix crash when trying to change VTE directory at startup

VTE: Fix crash when trying to change VTE directory at startup

2150302fe6e88f88e5eba78502b478be2b78c662 introduced a bug, because
configuration_load_session_files() calls VTE code if vte_info.have_vte
is non-FALSE, before vte_init() has been called.  So, it relied in the
implicit 0-initialization of the vte_info global, which the above
commit changed carelessly.

So, instead of altering vte_info.have_vte early, add a specific flag
for whether VTE is disabled on the command line.

Also, explicitly initialize vte_info to suggest to the reader the
default values does matter.